There are several major processes known in the art for recovering hydrocarbons from dehydrogenation processes or lean refinery-type gas mixtures. One such method is oil scrubbing, wherein heavy hydrocarbons are absorbed from the feed gas by a circulating heavy oil in an absorber column and are then recovered in a stripping column. The non-condensable vapor overhead from the stripping column may be recycled back through the absorber column for enhanced recovery. An example of this type of process is described in U.S. Pat. No. 3,274,102.
A second method for recovering hydrocarbons from a natural gas or similar refinery or process stream is by cryogenic partial condensation. As described in U.S. Pat. No. 4,140,504, the gas is cooled at high pressure to produce vapor and liquid portions. The liquid portion from the partial condensation is further cooled and then expanded to a lower pressure. At the lower pressure, the liquid is supplied to a distillation column, where it is separated into fractions. The vapor portion is work-expanded to the operating pressure of the distillation column and supplied to the distillation column below the feed point of the expanded liquid portion. The liquid product is then revaporized to provide refrigeration for the system. The low pressure product gas is compressed and reliquified before it is recovered.
Two processes for recovering hydrocarbons from a feed gas using dephlegmator cycles are disclosed in U.S. Pat. Nos. 4,002,042 and 4,270,940. In U.S. Pat. No. 4,002,042 a feed gas containing C.sub.2 + hydrocarbons and lighter components is introduced to a dephlegmator to effect its separation into a vapor stream and a condensate stream. An extraneous refrigerant, such as ethylene, is evaporated in the dephlegmator to provide refrigeration for the system. The condensation stream is then passed to a demethanizer column where it is fractionated into an overhead methane-hydrogen stream and a bottom product ethylene-ethane stream. In this process a large fraction of the C.sub.2 + is initially recovered upstream of the dephlegmator in a heat exchanger, or in a series of heat exchangers, as in a typical ethylene plant.
U.S. Pat. No. 4,270,940 discloses a dephlegmator cycle for recovering C.sub.2 hydrocarbons. In this process enhanced recovery of ethane and ethylene from demethanizer column overhead is obtained by subjecting the uncondensed vapor effluent from the main reflux condenser to further condensation and accompanying rectification in a dephlegmator and returning the liquid condensate from the dephlegmator to the demethanizer column. A large portion, e.g., 95% or more, of the C.sub.2 hydrocarbons are separated initially in the demethanizer column before the uncondensed vapor from the main reflux condenser enters the dephlegmator. The C.sub.2 + hydrocarbons which remain in the main condenser effluent vapor stream are condensed in the dephlegmator and the liquid condensate is then recycled back through the demethanizer to obtain the final high C.sub.2 recovery. The liquid condensate recovered from the dephlegmator is of low purity, e.g. 5 to 10 mole % C.sub.2 +, and must be fractionated in the demethanizer to remove the 85% or more CH.sub.4 impurity.